R1:f6d4cc4896ce

Send custom headers for author, reviewer and cc in Differential e-mails

Summary:
I want to flag messages which require an immediate action from me in e-mail
client.
It is currently not possible because Author and Reviewers fields are both in
To:.
So the filtering rule cannot recognize if I am the person who should take the
action.

This diff adds these headers:

- X-Differential-Author
- X-Differential-Reviewers
- X-Differential-CCs

Test Plan:
Send comment to the diff.
Verify X-Differential-* headers.